Fermented sour soup is a unique traditional sour food of the Miao and Dong ethnic groups in China. However, due to the commonly used traditional fermentation, the unstable fluctuation of microbial community often leads to some unpleasant inherent off-flavours in rice sour soup. In this study, the microbial composition and volatile flavour components of off-flavour and normal rice sour soup were examined by high-throughput sequencing and headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry. Then, based on Pearson correlation coefficients, the correlations between fungi and bacteria and those between core microorganisms and key volatile flavour components were investigated. The dominant fungal genera included Candida, Dekkera, Pichia, Rhizopus, and Issatchenkia, whereas the dominant bacterial genera included Lactobacillus, Pectinatus, Bifidobacterium, Pseudomonas, and Acetobacter. Compared to normal rice sour soup, off-flavoured rice sour soup contained the significantly increased relative abundance of Candida and the significantly decreased relative abundance of Dkkera. In addition, 80 volatile flavour compounds detected in off-flavoured rice sour soup, mainly including esters, acids, and alcohols, were different from those detected in normal rice sour soup. Especially, some substances were only detected in off-flavoured rice sour soup, such as 1-hexanol (68–207 μg/kg), butyric acid (20–92 μg/kg) and heptanoic acid (12–45 μg/kg), which might be the main source of off-flavours. These results provide new ideas and strategies of removing or abating odour from fermented rice sour soup.

Compared with the rice-acid soup inoculated with single starter, the synergistically intensified rice-acid soup inoculated with Lactobacillus paracasei H4-11 (L. paracasei H4-11) and Kluyveromyces marxianus L1-1 (K. marxianus L1-1) contained more flavor compounds. Organic acids mainly included L-lactic acid and the main volatile flavor component was ethyl acetate. Moreover, the signal intensity of astringency and bitterness and the total concentration of volatile sulfur compounds were reduced. The combined analysis results of RNA sequencing (RNA-seq) technology and 4D label-free quantitative (4D LFQ) proteomics explained the flavor formation pathways in rice-acid soup inoculated with L. paracasei H4-11 and K. marxianus L1-1. In L. paracasei H4-11, L-lactate dehydrogenase, phosphoglucomutase, acetate kinase, alcohol dehydrogenase and acetyl-CoA were up-regulated and D-lactate dehydrogenase and N-Acetyltransferase were down-regulated. In K. marxianus L1-1, Acetyl-CoA, acetaldehyde dehydrogenase, acyl-coenzyme A, N-acetyltransferase, and L-lactate dehydrogenase were up-regulated and hexokinase, alcohol dehydrogenase, and alcohol O-acetyltransferase were down-regulated. The above up-regulation and down-regulation synergistically promoted the formation of characteristic flavor compounds (mainly L-lactic acid and ethyl acetate). Enzyme-linked immunoassay (ELISA) and parallel reaction monitoring (PRM) quantitative analysis respectively verified that 5 key metabolic enzymes and 27 proteins in L. paracasei H4-11 and K. marxianus L1-1 were associated with the characteristic flavor of rice-acid soup, as confirmed by the quantitative results of 4D LFQ.

The physicochemical properties and composition of coix seed oil produced by Monascus purpureus fermentation and supercritical CO₂ extraction were determined. Anti-lipid-oxidation and edible safety were evaluated using a cholesterol-fish oil model, acute oral toxicity assay, and genetic toxicity assay in vitro and in vivo, respectively. The results show that the extraction oil from fermented coix seed (FCS-O) had good physicochemical quality and abundant active components with physiological function. In particular, γ-tocotrienol, γ-oryzanol, coixenolide and oleic acid concentrations reached 72.83 µg/g, 745.96 µg/g, 9.65mg/g and 316.58 mg/100 g DW, respectively. The FCS-O exhibited higher antioxidant capability in inhibiting lipid oxidation and peroxidation. Compared to the blank control, the concentrations of 7-ketocholestreol and peroxide only were 8.42 µg/mL and 16.16 mmol/kg at 168 h of oxidation (P < 0.01). In addition, the FCS-O has been confirmed to be a very safe edible oil, with no acute toxicity (LD₅₀ > 10 g/kg bw, considered actually non-toxic) and no induced mutagenicity, cytotoxicity or genotoxicity. These results serve as a good safety reference for future application of the oil from fermented coix seed. The development and utilization of this kind of oil will be beneficial as a food, food ingredient, nutritional supplement, or natural food antioxidant to promote good health function.

This study aims to explore the formation mechanism of ethyl acetate and organic acids in acid rice soup (rice-acid soup) inoculated with Kluyveromyces marxianus L1-1 through the complementary analysis of transcriptome and proteome. The quantity of K. marxianus L1-1 varied significantly in the fermentation process of rice-acid soup and the first and third days were the two key turning points in the growth phase of K. marxianus L1-1. Importantly, the concentrations of ethyl acetate, ethanol, acetic acid, and L-lactic acid increased from day 1 to day 3. At least 4231 genes and 2937 proteins were identified and 610 differentially expressed proteins were annotated to 30 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways based on the analysis results of transcriptome and proteome. The key genes and proteins including up-regulated alcohol dehydrogenase family, alcohol O-acetyltransferase, acetyl-CoA C-acetyltransferase, acyl-coenzyme A thioester hydrolase, and down-regulated aldehyde dehydrogenase family were involved in glycolysis/gluconeogenesis pathways, starch and sucrose metabolism pathways, amino sugar and nucleotide sugar metabolism pathways, tricarboxylic acid (TCA) cycle, and pyruvate metabolism pathways, thus promoting the formation of ethyl acetate, organic acids, alcohols, and other esters. Our results revealed the formation mechanisms of ethyl acetate and organic acids in rice-acid soup inoculated with K. marxianus L1-1.

Rice-acid, a Chinese traditional acidic rice soup (rice-acid), is widely accepted by consumers due to its unique flavor and anti-oxidation, anti-aging and immunity enhancement functions. This study confirmed that L-lactic acid and malic acid were the main organic acids in rice-acid. Low-temperature rice-acid samples produced by enterprises had the highest signal intensity of sour taste. The total content of free amino acids in different fermented rice-acid samples were in the range of 0.003–0.468 mg/g. 42 key volatile flavor compounds were identified in rice-acid. 8 volatile compounds with a higher contribution to the aroma of rice-acid were respectively acetic acid, 1-octen-3-ol, 2-heptanol, ethyl acetate, propyl propionate, hexanal, nonanal, and 2,3-butanedione. The interaction between lactic acid bacteria (3.00×10³–7.02×10⁶ CFU/mL) and yeasts (5.04×10⁴–2.25×10⁸ CFU/mL) affected the formation of taste and aroma components in rice-acid. The physicochemical characteristics including titratable acidity, pH, reducing sugars, amino acid nitrogen, gamma-aminobutyric acid showed significant differences between low-temperature fermentation samples and high-temperature fermentation samples. In addition, relationships linking all data through Pearson coefficient correlation were also reported. In summary, the study can be used to improve the quality of rice-acid products.